Offset printing plates generally define two surface conditions: an oily ink receptive/fountain solution (or dampening water) repellent condition defining the pattern to be printed and an oily ink repellent/fountain solution (or dampening water) receptive condition. Fountain and dampening solutions are mildly acidic solutions used to keep the non-imaging areas of the printing plate free from scumming or tinting, i.e., free from becoming ink receptive.
An image produced by an offset printing plate can be altered by changing the surface from one condition to another. U.S. Pat. No. 4,396,703 teaches an acidic retouching agent which acts to delete an image area from a lithographic printing plate having a hydrophilic surface. A viscous fluid is applied to the areas of the plate on which an image is to be deleted. It will be appreciated that application of a liquid is not easily controlled and is not suitable for detailed corrections.
In a process for adding image areas to a metal printing plate, an electrical arc is produced between a graphite electrode and the plate, where the arc first scores the plate and then deposits carbon on the etched surface area to form additional oily ink receptive areas. This arcing retouch process produces inconsistent results, both in initial image quality and image longevity, i.e., the number of print impressions an image area can produce before it begins to lose its oily ink receptivity with the printed image appearing progressively weaker and/or faded on each successive print impression. There is also a concomitant loss of productivity due to the time required in setting up and removing the graphite anodes and the need to remove the plate from the printing press in the preferred process. Further, the transformer, wires, connectors, etc. associated with the device provide a cumbersome apparatus.
There are also corrections which cannot be made. For example, a straight line cannot be accurately drawn since the electrical arcing between the pencil and the plate has the deleterious effect of scoring the plate in areas outside the area to be modified, rendering those areas to be oily ink receptive and ruining the entire plate.
In yet another process for adding image areas to a printing plate, a caustic liquid is dispensed from a cylindrical container with a felt or similar material tip, such as is commonly used for colored marking pens. The liquid dries on the surface of the printing plate, to produce an oily ink receptive area. The effects of the caustic liquid are also inconsistent in both initial image quality and image longevity. Further, the caustic liquid produces a strong, disagreeable odor and there is some danger associated with the use of a caustic liquid. Due to liquid spreading, it is difficult to use the process for intricate or detail work or where an amount of image bleed or travel is unacceptable.
These and other problems are overcome by the present invention and an improved material is provided in a process for adding image areas to offset printing plates.